Photo-imageable coverlays (PICs) provide a solution for mass production of flexible HDI (high-density interconnect) boards. PICs are similar to solder masks used in rigid printed circuit boards (PCBs), but as a material for flexible PCBs (FPCBs), they must have certain levels of flexibility.

There are two types of PIC: liquid photo-imageable ink (LPI) and dry film. In the wet process, the LPI is evenly coated on the circuits using screen printers, bake-dried, exposed and developed to obtain soldering point patterns. In the dry process, the dry film is coated with a layer of ductile solder mask and then laminated onto the FPCB using low-temperature vacuum pneumatic thermal laminators. The general manufacturing process of flexible HDI is complicated - which involves material cutting, CNC drilling, through-hole plating, dry-film lamination, exposure, etching, hole pre-punching, AOI (automated optical inspection) testing, pre-lamination, LPI screen printer coating, electric treatment, surface mounting, punching, function testing and quality control. However, using PICs can significantly simplify the process.

Dry-film lamination solves problems of clogged through-holes

For double-side and multi-layer PCBs that need BGA insertion, LPI has to be used to meet precision requirements. However, the use of LPI unavoidably causes a problem, that is, through-holes in BGA areas will be clogged with ink. The problem can be easily solved by using dry-film PIC lamination, which can also significantly increase yield rates and production efficiency. Why are PICs important? It is because the use of PICs will change the way of producing FPCBs and therefore the ecosystem of Taiwan's FPCB industry. It will enhance Taiwan-based makers' competitiveness in the face of Japan- and US-based competitors in high-end FPCB market.

Taiwan-based makers developing materials for FPCB coverlays

There must be a cover layer to protect the circuit of a FPCB from the impacts of temperatures and humidity, as well as pollutants and corrosive substances. There have been three major types of cover layer: polyimide (PI) film, thermal laminated solder mask and photo-imageable solder mask. But now PICs are new materials that can replace both PI films and photo-imageable solder masks. The concept of PIC is not new, though. US-based DuPont developed such a prototype more than 10 years ago and then other firms in Japan and the US, including Hitachi Chemicals and Morton, also attempted to develop similar products but failed. However, FPCB makers have maintained a strong interest in PICs simply because the use of PICs provides a simpler and more efficient way of attaining high precision required for FPCBs.

PICs feature high precision and flexibility and offer a better solution than PI films in terms of flexibility and circuit precision on FPCBs. Taiwan-based Teamchem Materials has developed PICs and cooperated with FPCB makers in southern China. Teamchem is renowned for specialty chemical materials and has worked with various major FPCB makers in successfully developing specialty ink tailored made for the clients' needs. Teamchem is a champion in developing PICs, and Yeh says his company is ready to work with FPCB makers to jointly develop products that cater to their specific needs.

PICs developed and produced by Teamchem

FPCB makers in southern China replace PI films with PICs

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